1. Field of the Invention
The present invention relates to a control device for a variable displacement engine, especially for automobile, which varies the number of working cylinders depending on the driving condition.
2. Description of the Related Art
An engine which stops some of its cylinders working in a partial load driving condition to save fuel, is known as a variable displacement engine.
In the variable displacement engine, some of cylinders stop working, and only the remaining cylinders continue to work, in a partial load driving condition.
On the other hand, in the conventional engine, all the cylinders work all the time, even in a partial load driving condition.
Under the same load condition, to get the same power, the same quantity of air is required for both the variable displacement engine and the conventional engine. Therefore, the intake passages of the conventional engine must be throttled to a smaller size than the intake passages of the variable displacement engine.
Therefore, the intake air pressure, upstream of the intake valve, in the conventional engine is lower than that in the variable displacement engine, and the pumping loss in the conventional engine is greater than that in the variable displacement engine. As a result, the variable displacement engine can save the fuel compared to the conventional engine.
It is known in the related art to change the working cylinders without changing the number of the working cylinders, as disclosed in Japanese Unexamined Patent Publication No. 52-36230, for instance, at intervals of a predetermined number of cycles, or at a predetermined time, or by detecting that the temperature of a catalytic converter is lower than a predetermined value, to prevent a reduction of the life of an engine, due to the repeated cooling-heating cycle, and a reduction in the purifying ability of catalytic converter due to a fall in the temperature of the catalytic converter.
By the way, in the variable displacement engine, such as described above, a shock can occur even if the working cylinders are changed without changing the number of the working cylinders. Especially, in the case of variable displacement engine connected to an automatic transmission, a series of shocks may occur in succession. Because, if the driver accelerates or decelerates a vehicle after running at constant speed with a partial load, a shock occurs when the automatic transmission shift, and then, a shock occurs when the working cylinders are changed.